Jurnal Teknologi

Jurnal Teknologi

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STUDY ON PERFORMANCE OF CO2 LASER IN PAINT REMOVAL OVER SELECTED NATIONAL CAR MODEL

Article history Received 15 August 2015 Received in revised form 15 November 2015 Accepted 30 December 2015

Mohammad Khairul Azhar Abdul Razaba*, Mohamad Suhaimi Jaafarb, Fatanah Mohamad Suhaimic, Mohamad Najmi Masria, Nor Hakimin Abdullaha, Mazlan Mohameda, Mohamad Faiz Mohd Amina, Muhammad Iqbal Ahmada

*Corresponding author [email protected]

aAdvanced

Material Research Cluster, Faculty of Earth Sciences, Universiti Malaysia Kelantan Jeli Campus, Locked Bag No. 100, 17600 Jeli, Kelantan, Malaysia bSchool of Physics, Universiti Sains Malaysia Main Campus, 11800 Minden, Penang, Malaysia cAdvanced Medical & Dental Institute, Universiti Sains Malaysia, Bertam 13200 Kepala Batas, Penang, Malaysia Graphical abstract

Abstract A technique to determine the optimum parameters of 30 Watt Continuous Wave (CW) CO2 laser paint removal has been developed on national car coated substrate. This paper reports on the results of studies carried out on two different coating thicknesses; 196 µm and 201 µm using CW CO2 lasers operating at 10, 600 nm wavelength and relatively high beam power from 40%, 50%, 60%, 70%, 80% and 90% out of 30 Watt. Empirical data were presented to demonstrate the optimum power required for paint stripping process. Macro analysis was done to prove the stripping process was in line with the increase of power percentage applied, whereas microanalysis using EDX and SEM revealed the atomic composition and surface roughness of the crater. Keywords: CO2 decomposition

laser,

Paint

removal,

Car

coated

substrate,

Thermal

Abstrak Satu teknik untuk menentukan parameter optimum laser CO2 gelombang berterusan (CW) 30 Watt dalam penanggalan cat telah dilaksanakan terhadap substrat kereta bercat. Kertas penyelidikan ini melaporkan hasil kajian dua perbezaan ketebalan cat; 196 µm dan 201 µm menggunakan laser CW CO2 beroperasi pada 10, 600 nm panjang gelombang dan perbandingan kuasa alur bermula dari 40%, 50%, 60%, 70%, 80% dan 90% daripada 30 Watt. Data empirikal telah dibentangkan untuk menunjukkan kuasa optimum yang diperlukan bagi proses penanggalan cat. Analisis makro telah dilakukan untuk mebuktikan proses penanggalan cat adalah selari dengan peningkatan peratus kuasa yang ditetapkan manakala analisis mikro menggunakan EDX dan SEM memberikan komposisi atom dan kekasaran permukaan kawah. Kata kunci: CO2 laser, Penanggalan cat, Substrat kereta bercat, Penguraian termal © 2016 Penerbit UTM Press. All rights reserved

78:3 (2016) 203–209 | www.jurnalteknologi.utm.my | eISSN 2180–3722 |

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Mohammad Khairul Azhar Abdul Razab et al. / Jurnal Teknologi (Sciences & Engineering) 78:3 (2016) 203–209

1.0 INTRODUCTION Nowadays, chemical-based strippers and grit blasting are the core techniques practiced by Malaysian automotive industry in the process of repainting cars. However, neither of these techniques is ideal as both have resulted in environmental imbalance due to the generation of a large amount of waste and the process is unfavourable due to the high cost involved; especially the labour cost, [1-5]. Laser based paint removal processes can overcome these problems. The method is based on the removal of a thin layer of coating material from a metal surface under the action of the continuous wave (CW) laser beam, generally known as “laser thermal de-composition”, [6-8]. The main advantages of this technique are; there is no direct mechanical contact to the substrate, in situ cleaning, less time cleaning, non-use of toxic solvents or chemical products, preventable damage to the substrate and controllable environment, [9, 10]. However, although benefits of this technique are highly feasible, laser cleaning of painted surfaces can only obtain a good result if the process is properly controlled, [10, 11]. This paper demonstrates the optimum power operation condition of Synrad J48-1 CO2 laser 30 Watt applied to strip the paint of a national car body. This study aimed to introduce the fundamental of laser paint removal method as an alternative to conventional chemical stripping in Malaysian automotive industry and at the same time to prove its effectiveness and quality. The experiment was carried out at Medical Physics Laboratory, School of Physics, Universiti Sains Malaysia, Penang and Material Science Laboratory, Faculty of Earth Science, Universiti Malaysia Kelantan Jeli Campus.

2.0 EXPERIMENTAL 2.1 Laser Equipment Preparation

and

Substrate

Samples

Before the experiment started, the laser power output was calibrated with PW-250 Power Meter in order to ensure the output power was constantly increasing to the power set-up using UC-2000 PWM controller. In this study, two substrate samples with black paint colour were prepared from the right front door of selected national car model obtained from car workshop and spare part centres in Kota Bharu. The door was cut into small rectangular substrate, approximately 4 x 4 cm2 in size for SEM analysis availability. Before irradiation, both substrates provided were cleaned by running tap water to remove any contaminants and foreign particles. The average coating thickness of substrate sample was measured by using CEM DT-156 Paint Coating Thickness Gauge Tester F/NF Probe. Five readings were taken from each side and the centre of the

square face of the sample substrate. Each substrate was marked by a unique number at the backside for sample identification. Moreover, one substrate sample was prepared for three different power parameters with three times laser irradiation required by each substrate. Hence, eighteen times of laser irradiation was utilized on two pieces of substrate samples in order to complete the six selected laser power parameters. Low power was applied at the bottom of the sample and the next high selected power was irradiated at the centre and top of the substrate respectively. 2.2 Operating Principle Prior to irradiation, the sample substrate was fixed on the holder shown in Figure 1.

Figure 1 Car substrate sample was fixed on the holder for laser stripping process

For this study, source to target distance (STD) was fixed at 10cm by locking the sample holder on the ruler track provided. With different densities of the laser power, the substrate samples were then irradiated based on the power level marked at the backside of the substrate. The laser power applied for the purpose of completing one cycle assessment were 40%, 50%, 60%, 70%, 80% and 90% out of 30 Watt. These selected laser power were emitted by fixing the irradiation time at 120 seconds. During laser irradiation process, the sample was moved in horizontal line using the left-right mechanical controller equipped to the holder. This method enlarge the stripping area by continuously moving the substrate from left to right edge. The sample movement and laser irradiation will be stopped concurrently after 120 seconds. This process was repeated for three times with the same laser power in order to get the consistency and wide stripping area before proceeding with different selected laser power. Laser safety glass manufactured by Yamamoto was worn during the whole experiments for eyes protection from CO2 laser irradiation. 2.3 Sample Analysis The irradiated samples were cleaned by using plain water and dried in open air to remove any burning residue existed on the surface substrate. Cotton wool and tissue paper were used to swap the stripped area in order to ensure there were no more residues

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left and at the same time to keep the originality of the structure and pattern of the crater before proceeding to sample analysis. Macro analysis was done to find out the thickness of paint that has been removed in the laser stripping process. Electronic Vernier Digital Caliper (EVDC) and CEM DT-156 gauge coating tester were used to measure the paint thickness before and after the irradiation. In this case, EDCV measured the paint thickness together to the metal substrate, whereas CEM DT-156 only measured the paint thickness by applying magnetics resources. Hence, the depth of the paint that has been removed by different laser power will be revealed and analysed. In addition, each crater resulted from different laser power were viewed by using Meiji MT 7100 Metallurgy Microscope for micrograph comparison. Both irradiated samples were sent for micro analysis by using scanning electron microscopy (SEM) for surface morphology. Energy dispersive x-ray (EDX)

technique was used for atomic composition study. The surface morphology was magnified to 500 times with the scale of 100µm for substrate damage and roughness analysis. In addition, 20kv penetration power was applied for EDX analysis in order to get the accurate percentage of atomic composition presence on the surface and subsurface of irradiated area.

3.0 RESULTS AND DISCUSSION 3.1 Macro Analysis Macro analysis was carried out by using EVDC and CEM DT-156 gauge coating thickness tester to measure the depth of paint before and after the stripping process. The results produced by both instrument were compared and their relation to paint removal thickness were shown in Table 1.

Table 1 The thickness of paint removed by varying laser power measured by EVDC and CEM DT-156 coating thickness tester.

Laser Parameters

Electronic Digital Caliper results (µm) Plate + Plate + Paint Paint Residue Removed Paint (depth) 950 -

CEM DT-156 Coating Thickness Tester results (µm) Paint Residue Paint Thickness Paint Removed (depth) 196 -

Irradiation

PWM (%)

Time (sec)

STD (cm)

-

-

-

-

1

40

120

10

-

820

130

-

106

90

2

50

120

10

-

800

150

-

83

113

3

60

120

10

-

790

160

-

75

121

-

-

-

-

930

-

-

201

-

-

4

70

120

10

-

750

180

-

78

123

5

80

120

10

-

730

200

-

75

126

6

90

120

10

-

720

210

-

76

125

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Paint Removed VS Laser Power for EDVC & CEM DT-156 250

Depth of paint Removed (µm)

200

200

210

180 150

150 130

100

160

113

121

123

126

125

50

60

70

80

90

90

50 0 40

Laser Power (%) EDCV

CEM DT-156

Figure 2 Depth of paint removed for different laser power by using EDCV and CEM DT-156 techniques

Figure 2 shows the depth of paint removed which was in line with the increasing of laser power for both techniques, [12]. At 60% and next high-applied power, the paint thickness removed measured by CEM DT-156 was constant. This criterion might be due to the non-uniformity of the crater caused by high power laser irradiation which will lead to the fluctuation of the magnetic resources of the device. The non-uniformity surface morphology was proven by SEM micrograph as shown in Figure 6 (a) and (b). Dissimilar for EDCV result, the thickness of paint removed were constantly raised in accordance with the laser power applied. Flat surface at the backside of the metal substrate was supporting the EDVC nozzle when the measurement was taken hence the reading is more stable. The surface structure of the irradiated substrate samples were then viewed by using Meiji MT 7100 Metallurgy Microscope as shown in Figure 3 for initial surface morphology analysis. In accordance to Figure 3 (a), (b) and (c), the applied laser power at 40%, 50% and 60% were unable to remove the topcoat of the coating system. However, this problem was overcome by increasing the laser power at 70%, 80% and 90% as shown in Figure 3 (d), (e) and (f). Surface morphology revealed that at low laser power application has caused the surface roughness to be more critical compared to higher laser power. The micrograph shows the depth of paint that has been removed which was almost the same as well as their roughness level started at 70% and above power applied, hence support the graph pattern produced by using CEM DT-156 in Figure 2.

a.

b.

c.

d.

e.

f.

Figure 3 Meiji MT 7100 Metallurgy Microscope micrograph with 40x magnification lens for different laser power were applied in stripping process; (a) 40%, (b) 50%, (c) 60%, (d) 70%, (e) 80% and (f) 90%

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3.2 Micro analysis

non-irradiated and irradiated sample substrate. Table 2 below shows the result of EDX analysis done from 40% to 90% of selected laser power applied for stripping process.

Microanalysis was conducted using Energy Dispersive X-ray (EDX) to investigate the atomic composition for

Table 2 EDX results of atomic composition revealed (Wt %) for laser stripping process from 40%, 50%, 60%, 70%, 80% and 90% out of 30 Watt laser power

Element (Wt %) C N O Na Mg Al Si S Cl Ca Ti Fe Ba Total

Laser Power (%) *N/A

40

50

60

70

80

90

73.11 26.89 100

61.87 33.83 2.18 2.12 100

74.6 23.5 0.55 0.73 0.62 100

78.03 20.9 0.44 0.63 100

14.6 59.81 1.04 2.78 9.39 5.41 1.49 0.74 2.93 1.13 0.67 100

60.75 0.95 4.93 7.34 7.93 1.61 9.51 1.93 1.62 3.44 100

53.79 0.81 7.0 13.42 13.46 0.83 4.21 1.49 5.0 100

The basic composition existed at the painted surface or topcoat was carbon (C) and oxidized particle (O) which is typical for organic coatings, [13]. When the paint was removed, carbonization process occurred due to thermal decomposition [14]. This process started at 40% applied laser power where the exchange of C and O composition percentage can be seen in Table 2. Absence of C at 70% of power level and above proved that the topcoat layer is already stripped. The most important element to be considered is Ferum (Fe) as the main composition of metal substrate, which is justified for all coated area, has been removed. Since EDX result revealed the atomic composition at